Cryptosporidium is an intracellular protozoan parasite that causes diarrhea in immunocompetent patients and patients with AIDS. There is no treatment for cryptosporidiosis and very little is known about the pathogenesis of the disease. The goal of this project is the identification and characterization of cryptosporidial genes encoding sporozoite protein antigens which are the targets of antibody that inhibits parasite motility and/or prevents parasite attachment to, or penetration of, the host gut epithelial cell. Attainment of this goal will: (1) provide information and reagents necessary to rationally develop immunoprophylactic and/or passive immunotherapeutic strategies for potential intervention in human cryptosporidial disease, (2) identify Cryptosporidium proteins that have significant functional roles in host-cell invasion and, (3) provide immunological and recombinant DNA-based protein and peptide reagents with which to study the biology of invasion at the molecular level. The specific aims of this research project are: 1) to establish an ongoing neonatal calf propagation system for reliably obtaining large numbers of oocysts of the AUCP-l isolate of C. parvum, 2) to biochemically and immunologically identify and characterize proteins exposed on the surface of C. parvum sporozoites, 3) to prepare polyclonal antisera and monoclonal antibodies against sporozoite and oocyst proteins and quantitatively test these reagents for their ability to inhibit sporozoite invasion of MDCK cells in vitro. 4) to molecularly clone, isolate and sequence genes/cDNAs encoding C. parvum proteins that are the targets of invasion-inhibiting antibody and/or that reside on the surface of sporozoites.